Depth estimation of an arbitrary shape delamination in composite laminate using modified LDR frequency based on acoustic measurement

Accurate depth determination for arbitrary shaped delamination remains a challenge in local defect resonance (LDR) testing methodology due to unavailability of simple LDR frequency expression for arbitrary delamination. In this paper, a modified LDR expression for arbitrarily shaped delamination has been proposed for accurate delamination depth determination based on area equivalence. Further, selection of best LDR‑based non-destructive testing techniques among laser Doppler vibrometry (LDV), vibro‑thermography (VT) and acoustic testing has been carried out for depth determination. Two CFRP specimens were fabricated using the Vacuum Assisted Resin Transfer Moulding (VARTM) process. A flat bottom hole (FBH) was introduced in the first specimen, while barely visible impact damage (BVID) was generated in the second specimen using a portable impact testing setup. The internal damage was characterised using phased array ultrasonic testing (PAUT) and X-ray computed tomography (CT), revealing multi-layer delamination. Subsequently, LDV, VT and acoustic testing were applied to the impacted specimen. All three techniques successfully identified the two-layer delamination along with the protrusion. Among these, acoustic testing proved to be the fastest and most accurate NDT method. Furthermore, the defect depth estimated using the modified LDR frequency‑based acoustic testing showed good agreement with the results obtained from PAUT.